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Differential Impedance Measurement

Given today's high-speed RF and digital integrated circuits run at reduced voltages, noise immunity is an important factor in any interconnect system. Differential signaling is a convenient way to avoid coupling common-mode noise into a high-speed transmission line.

Since differential signaling involves two wires plus a ground reference, two transmission modes exist: the common mode and the differential mode. These two modes generally have two different propagation velocities (for microstrip) and the characteristic impedance is a 2x2 matrix. Loads and sources can excite common (even) and differential (odd) modes. This complicates the characterisation of the transmission lines and the de-embedding of load impedances.

The goal of this work is to extract the common mode and differential mode impedance of an RF integrated circuit from a printed-circuit board based transmission line system. The article at this link describes the method in detail.

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This text has not been peer reviewed, but I have tried to eliminate any errors. Readers are urged to check the results of any measurements that they attempt using the methods that are described in this paper. Please feel free to contact me with any questions or comments.


Last updated: 20091023